1. Field of the Invention
The present invention relates to content addressable memory, and more specifically a high density content addressable memory cell utilizing phase change material and an access device.
2. Description of Background
Content addressable memory (CAM) is a type of computer memory utilized in high speed searching applications. Most CAM devices utilize static random access memory (SRAM) as data storage devices (utilizing transistors to store memory), and additional transistors and complementary transistors for match operations. Typically in these CAM devices search-line access elements and word-line access elements are necessary to operate and program individual memory cells in the memory arrays. The search-line access elements and word-line access elements are often comprised of power intensive large drive field effect transistors (FET).
Phase change material can also be utilized to store information in CAM devices. Phase change materials can be manipulated into different phases or states, with each phase representing a different data value. Generally, each phase exhibits different electrical properties. The amorphous and crystalline phases are typically two phases used for binary data storage (1's and 0's) since they have detectable differences in electrical resistance. Specifically, the amorphous phase has a higher resistance than the crystalline phase.
Chalcogenides are a group of materials commonly utilized as phase change material. They typically contain a chalcogen (Periodic Table Group 16/VIA) and a more electropositive element. Selenium (Se) and tellurium (Te) are the two most common semiconductors in the group used to produce a chalcogenide when creating a phase change memory cell. Examples of this type of material are Ge2Sb2Te5 (GST), SbTe, and In2Se3. However, some phase change materials do not utilize chalcogen, such as GeSb. Thus, a variety of materials can be used in a phase change material cell as long as they can retain separate amorphous and crystalline states.
Two problems in phase change CAM devices and standard CAM devices are the large size of individual memory cells in the memory devices (requiring complementary access transistors and complementary memory elements) and the large power required to perform search operations in the memory devices (requiring word-line charging using large, power intensive drive FETs). Thus, it is desirable to devise a method to reduce the size of the individual memory cells and to eliminate word-line charging.